The local-regional relationship: immigration, extinction, and scale

While local processes (e.g., competition, predation, and disturbance) presumably cause species exclusion and thus limit diversity in individual communities, regional processes (e.g., historical events, immigration, and speciation) are assumed to provide a source of species to colonize and thus enrich local communities. Ecologists have attempted to distinguish between these two sets of processes using graphical evidence for local assemblage saturation. However, such efforts have been controversial and are antithetical to the fact that local diversity bears an imprint of both. We examine the local-regional species richness relationship from the perspective of the theory of island biogeography and develop a model that can produce the full range of observed local-regional richness relationships from linear to curvilinear. Importantly, unlike previous models, we do not require species interactions to produce the curvilinear pattern. Curvilinear relationships arise if per-species stochastic extinction rates are substantially higher than colonization rates, while linear relationships result if colonization rates are higher than extinction rates. Because we also show that merely changing the sampling scale can make local-regional relationships appear either saturated or unsaturated, an inference of ecological processes, derived solely from local-regional relationships, is unwarranted.